The Rail, Maritime and Transport Union now claims it has uncovered an incident report which suggests a fault had already been discovered in the two units involved in the accident before they were returned to service.

They were under the care of maintenance operator Arriva Traincare, who reported that unit 150211 had a fault involving the 'uncoupling lever pivot head', leading to an 8mm gap on unit 150203 and a 5mm gap on 150211.

The union has now demanded a full investigation into what it has described as 'inherent failures leading up the incident that raise serious concerns over the safety culture within Arriva Traincare'.

RMT general secretary Mick Cash said:

“The incident last week in Leeds raises serious questions about the safety culture at Arriva Rail North and Arriva Rail UK’s maintenance division – Arriva Traincare. RMT has warned in the past that penny-pinching at Arriva Traincare has put staff under impossible strain and there should now be a full, transparent and independent investigation into their operations in light of this incident.

“No one should be any doubt that the incident also puts the safety critical role of the guard in the spotlight yet again in terms of dealing with evacuations and reinforces the call for Arriva to scrap plans to axe guards from their Northern trains.

“RMT will not allow the deadly serious issues raised by the Leeds incident to be swept under the carpet.”

An Arriva Traincare spokesman said:-

"The safety of our staff and customers is our top priority. The maintenance of trains for our customers is carried out by highly-skilled and experienced engineers in accordance with the required specifications, and we follow all engineering maintenance procedures to the letter. With regards the incident at Leeds last week, a full and thorough investigation is now underway to identify the root cause of the incident. We await the findings of that report.”